1. Field of the Invention
This invention relates to pneumatic radial tires, and more particularly to a pneumatic radial tire having a light weight, small rolling resistance and excellent steering stability and durability of belt cords and the like. It is accomplished by using in at least one belt reinforcing layer cords obtained by twisting filament-resin composite bodies.
2. Disclosure of the Related Art
In radial tires having a steel belt, the steering stability, wear resistance and the like are very excellent because the belt rigidity is fairly high as compared with the case of using organic fiber cords such as rayon, polyester or the like in the belt. On the other hand, these tires have several problems in that the amount of steel used is large which degrades the rolling resistance, the weight of the belt is large and the centrifugal force during the running becomes large to cause standing wave and degrade the high-speed durability. Also, and rust is created by the penetration of water and the like from exterior to cause tire breakage.
In order to solve these problems, however, when so-called multifilament twisted fiber cords such as aramide fiber or carbon fiber having a high tensile modulus are used as a belt material, the steering stability and wear resistance are improved as compared with the case of using rayon or polyester fiber cords. But, this cannot be said that they are sufficient as compared with the case of using steel cords. Since the multifilament twisted fiber cord is a set of very fine filaments (diameter: about 5-35 .mu.m) and is very flexible, it has a modulus of elasticity fairly sufficient to tensile strain, but shows very low compressive modulus and bending modulus against compression strain and bending strain. Therefore, the bending rigidity of the belt in the ground contacting area during the turning is insufficient, so that a sufficient cornering force is not caused and the steering stability is poor.
On the other hand, Japanese Patent laid open No. 63-134310 and No. 63-151505 have proposed the use of fiber reinforced material obtained by impregnating a bundle of high-strength and high-modulus fibrous filaments with thermosetting resin or thermoplastic resin as a belt material having a light weight, an excellent rolling resistance and a sufficient steering stability. When such a fiber reinforced material is used as a belt material of a tire without twisting the filaments, it has a certain compression modulus, so that the steering stability is surely improved. However, this material has been confirmed to have a serious drawback on the durability that cords in the belt buckle at the bending compression side due to the buckling of the ground contacting rear region produced in the rapid turning of the tire and finally arrive at the occurrence of cord breakage after the repetitive bending. Such cord breakage is caused in the groove bottom of the ground contacting portion of the tread. Particularly, significant cord breakage are occurs even under a relatively mild condition the cord breakage is not caused in the steel cords usually used in the radial tire.
Furthermore, it has been confirmed that when the above filament-resin composite body is used as a twisted cord in the belt of the radial tire, if the cord strength, elongation at break and end count are unsuitable, local breakage of several cords in the belt is caused inside the tire in the riding over stones scattered on road surface or protrusions on the road, which becomes a safety problem. In order to prevent the occurrence of such breakage, however, the end count of the cords in the belt is increased to enhance the belt strength, but it is apt to create separation failure at the belt end.